If it part of your buisness yes you can. so technically if you declare someof the income from selling say frags you should be able to clain the expenses related to producing your income (from tax point not taking into account any issues in term of other law regarding livestock sale)

For your home aquarium, if its a substantial part of your income. Does not have to be your sole income.
But you need all the supporting paperwork. But it will be a mission to convince the taxman.

Work aquarium, like in an entrance area or waiting room. To keep waiting customers "entertained". Maybe then yes.

Couple of years ago it would have been easier. Taxman clamped down on a lot. My Car and Topcar magazine subscriptions I deducted from tax, 15 years ago. When I still worked in the car financing industry. When asked I replied that I need to know whats going on in my clients business environment and whats up with their competitors. While I'm in IT. Taxman accepted that.